1. Field of the Invention
This invention relates to a camera having an electronic strobe flash unit which charges in response to movement of a camera release member for effecting an exposure operation. More particularly, the invention assures a predetermined minimum amount of charging of the flash unit, prior to an exposure, independently of the actuation speed of the release member.
2. Description Relevant to the Prior Art
Many commercially available electronic strobe flash units have a manually operated on/off switch for charging the flash unit. When a camera operator wishes to use the flash unit, the switch is turned on, which connects a low-voltage battery to a DC to DC converter to charge the flash unit to a relatively high voltage. Should the operator fail to turn the switch off while the flash unit is idle, the converter continues to maintain the flash unit in a charged condition, thereby wasting battery energy. However, if the switch is turned off after the flash unit is fully charged, the unit will not remain charged indefinitely. It may fail to provide adequate illumination if considerable time elapses before the next exposure, unless the photographer remembers to again charge the unit. In this case, the previous charging energy is wasted and, even if the photographer remembers to recharge the unit just prior to exposure, a prized photograph may be missed because of the time lost in performing this function manually.
Various approaches to solving this problem have been directed to causing the flash unit to charge when the imminent use of the camera is predicted by some activity of the photographer.
U.S. Pat. Nos. 3,296,949 and 4,189,222 describe cameras having a power switch for a built-in electronic strobe flash unit. The switch is closed automatically, to cause charging of the flash unit, when a dust cover is moved to uncover a camera taking lens. When the camera is idle, battery energy is wasted unless the operator remembers to cover the lens.
U.S. Pat. No. 4,085,414 describes apparatus for automatically turning on and off an electronic strobe flash unit as a function of whether an associated camera is held in position in anticipation of taking a photographic picture. However, unless relatively elaborate multiple means are employed to sense vibration, gripping force etc, in addition to camera position, it is quite likely that the charger will remain activated when not desired because of the storage position of the camera, e.g. on a car seat, etc.
As an alternative approach to the same problem, it is also known in the prior art to provide for charging an electronic strobe flash unit in response to pre-exposure movement of a camera shutter-release member so that the flash unit is only charged when its associated camera is to be operated. For example, U.S. Pat. No. 3,651,372 describes a camera and a flash unit in which a camera operator is supposed to voluntarily delay final movement of the release member, which opens the shutter, until a flash-status lamp signals that the flash unit is sufficiently charged.
U.S. Pat. Nos. 3,953,864 and 4,007,469 describe other cameras having built-in flash units which are charged in response to pre-exposure movement of a shutter-release member.
In the above prior art employing pre-exposure movement of the shutter release member, a rapid uninterrupted stroke of the release member causes an exposure to occur without flash or an insufficient amount of flash illumination. For a flash exposure, actuation of the shutter-release member must be interrupted to allow for flash charging.
U.S. Pat. No. 4,239,361 describes a camera having a fast-charging flash unit which is charged between the time a camera shutter-release member is initially actuated and the time an exposure commences. The camera has a shield which is moved from a normal viewfinder-covering position to a viewfinder-uncovering position in response to actuation of the release member. As the shield is moved, the flash unit commences to charge, even before the operator can view the scene in the viewfinder. The operator would not depress the release member further to make an exposure until the scene has been framed. For most picture-taking situations, the framing time requires a few seconds, which are adequate to allow the flash unit to fully charge. Although an exposure can be made once the scene is framed, without waiting for the flash unit to charge, the camera operator could override the flash charging system if the release member is jabbed or stabbed.
U.S. Pat. No. 4,084,167 describes camera structure for charging a built-in strobe flash unit when a shutter-release member is actuated. If scene brightness is low, an electromagnet latches the release member in a pre-exposure position while the flash unit charges. Once the flash is charged, the electromagnet is actuated. This frees the release member for final movement by the camera operator to open the shutter. This camera structure would avoid taking an exposure without necessary flash.
However, interrupting movement of the release member to wait for flash charging is annoying to both a photographic subject and a camera operator. A longer indefinite posing time is required and the additional uncertainty as to when an exposure is to occur makes it more difficult for a subject to maintain the proper pose.
It is also known in the prior art to provide for charging an electronic strobe flash unit in response to actuation of a camera shutter-release member, without appreciably delaying execution of an exposure operation. U.S. Pat. No. 4,182,561 describes a camera having a fast-charging flash unit which is intended to be charged in response to a continuous uninterrupted stroke of a release member, during a brief interval of time between initial actuation of the member, which sets the camera for an exposure, and final actuation of the release member, which causes an exposure. The flash unit employs a 120-volt battery for charging a flash-firing capacitor directly (without a DC to DC converter) during pre-exposure operation of the camera. A battery of this type is not readily suitable for use with a compact electronically operated camera having low-voltage integrated circuits. Thus, a separate battery would be needed for the camera.